It's not possible unless you have static DNS. dyndns I don't think can support this. If you want to relay mail you'll need to use your ISP's mail relay as they have a static IP and properly configured MTA. Not sure you can do the same on a dynamic IP.

Looking for new beta users -> Co-Founder of Acompli. Previously worked at Zimbra (and Yahoo! & VMware) since 2005.

Ptr

We have a static DNS IP and we still had to have our service provider do this. Just call them up and tell them you need a PTR record pointed to a public IP for your Zimbra server. The change takes anywhere from 24 to 48 hours. Ours changed within 24.